[llvm-exegesis] Fix D80610.

Summary:
Using a .data() member on a StringRef was discarding the StringRef
size, breaking llvm-exegesis on machines with counter sums (e.g.
Zen2).

Reviewers: oontvoo

Subscribers: mstojanovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80982